Dynamic Bidirectional Coupling of Membrane Morphology and Rod Organization in Flexible Vesicles

The ordering of rod-like particles in soft, deformable containers emerges from the interplay of anisotropic interactions, geometric confinement, and boundary compliance. This competition couples internal particle organization to container morphology and is central to biological processes such as cell motility, division, and encapsulation, in which cytoskeletal filaments confined by lipid membranes actively reshape cells. Using a minimal model combining experiments and simulations of colloidal rods encapsulated in lipid vesicles, we show that soft confinement drives a bidirectional coupling between internal order and vesicle shape. This interplay gives rise to a phase diagram in which elongated vesicles promote nematic alignment at lower packing fractions, whereas higher packing fractions induce smectic-like ordering that reshapes vesicles into plate-like morphologies with increased bending energy. Furthermore, by controlling vesicle volume and membrane area, we demonstrate that these boundary conditions enable reversible tuning of both vesicle shape and internal rod organization. This study establishes a promising route for dynamically controlling colloidal self-assembly in soft containers and for mimicking ordering in cell-like compartments.

💡 Research Summary

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This paper investigates the bidirectional coupling between the internal ordering of rod‑like colloidal particles and the shape of the flexible lipid vesicles that contain them. By encapsulating fluorescent silica rods (average length ≈ 4.5 µm, diameter ≈ 1 µm, aspect ratio ≈ 4.5) inside giant unilamellar vesicles (GUVs) using a droplet‑transfer method, the authors obtain a tunable experimental platform in which both the vesicle volume and membrane area can be varied. Two‑color confocal microscopy provides three‑dimensional reconstructions of the vesicle boundary and the rod positions, allowing the authors to compute two key dimensionless parameters: the reduced volume ν = V₍ves₎/V₍sph₎ (a measure of deviation from a perfect sphere) and the packing fraction η = N V₍rod₎/V₍ves₎ (the fraction of vesicle volume occupied by rods).

Complementary coarse‑grained simulations are performed with LAMMPS. Each rod is modelled as a rigid body of five overlapping spheres (aspect ratio ≈ 5) and the membrane as a meshless, curvature‑elastic sheet. An explicit solvent exerts an osmotic pressure that fixes the vesicle volume, while the number of membrane particles controls the surface area. The same WCA repulsive potential governs all pairwise interactions, ensuring that the simulated system faithfully reproduces the experimental steric constraints.

Mapping experimental and simulated data onto the (η, ν) plane yields a comprehensive phase diagram with three distinct regimes. At high ν (near‑spherical vesicles) and low η, rods remain isotropically oriented. As ν decreases—i.e., the vesicle elongates—nematic ordering appears at packing fractions far lower than those required in bulk suspensions. This geometry‑induced alignment stabilises a nematic director along the long axis of the vesicle. At still higher η, the rods form smectic‑like layers that run parallel to the vesicle’s long axis. The emergence of smectic order exerts strong stresses on the membrane, driving a transition from elongated (linear) to plate‑like vesicle shapes. Plate‑like vesicles display a flattened morphology with high curvature localized at the rod tips and low curvature across the rod‑aligned faces.

The authors quantify the energetic cost of these shape changes by comparing the bending energy E_b of rod‑filled vesicles with that of reference vesicles containing only spherical particles (E_b,sph) at identical η and ν. While E_b/E_b,sph≈1 throughout most of the diagram, it rises sharply (≈1.1) in the coexistence region where linear and plate‑like vesicles compete, indicating that plate‑like morphologies are energetically expensive but nonetheless stabilized by the internal smectic stress.

A detailed curvature‑orientation analysis reveals that rods in high‑curvature tip regions adopt homeotropic anchoring, whereas rods on low‑curvature mid‑sections lie planar with respect to the membrane. This mirrors curvature‑dependent anchoring observed for smectic rods in rigid 2D elliptical wells, where a critical curvature c*≈1/(1.6 L) marks the transition. In large vesicles (N≈200 rods) the same threshold is observed, but in smaller vesicles (N≈30–100) the transition occurs at lower curvatures, suggesting that global shape constraints modulate the local anchoring behavior.

Finally, the study demonstrates reversible control over both particle ordering and vesicle shape by independently tuning vesicle volume (via osmotic pressure) and membrane area (by adding or removing lipids). By moving along trajectories in the (η, ν) space, the system can be switched repeatedly between isotropic‑spherical, nematic‑linear, and smectic‑plate‑like states. This establishes a versatile route for dynamically steering colloidal self‑assembly inside soft containers, with direct relevance to cellular processes such as cytoskeletal remodeling, artificial cell design, and soft‑matter actuation.

In summary, the work provides a quantitative framework for understanding how flexible boundaries and anisotropic inclusions co‑evolve, highlighting the importance of bidirectional coupling in soft biological and synthetic systems.